MG Car Repair: MGB core plug size, frost plugs, core plugs

Question
On my 1980 MGB how many core plugs (known here as frost plugs) are there and what size are they? I have a leak at the back of the engine and I wonder if that's the problem.

The Moss catalogue does not state the size.

Answer
Hi Ron.  There are 4 core plugs on the 18V engine block.  They are all Imperial size, 1 5/8 inches.  There is no suitable metric equivalent.  

Yes, there is one at the back of the block and they regularly rust through.

The most common leak on an 18V block is at the head gasket joint, immediately above the distributor.